In an image forming apparatus provided with both side copying and overcopying functions, a copied paper is fed into a paper stacking unit (40) before being fed again. In the conventional paper stacking unit, since the unit is disposed horizontally, there exists a problem in that paper is no well aligned, thus resulting in paper jamming, double paper feeding, etc. To overcome this problem, the paper stacking unit (40) is disposed with an inclination angle paper feed-out side up. For further improving paper feed reliability, the stacking unit (40) includes a paper lifting device (50), a paper side alignment device (60), and a paper curl correcting device (70).